Voltage Drop Formula for Electricians — NEC Guide with Worked Examples

The Voltage Drop Formula

The standard formula for voltage drop on a single-phase copper circuit is:

To find the voltage drop percentage — the number compared against the NEC's 3% recommendation — divide the voltage drop by the circuit voltage and multiply by 100:

For three-phase circuits, replace the factor of 2 with 1.732 (√3):

What Is K = 12.9 and Where Does It Come From?

The constant K = 12.9 is the DC resistivity of copper at 75°C, expressed in units that make the formula work out directly in volts when L is in feet and CM is in circular mils. More precisely, K = 12.9 Ω·cmil/ft means that a copper conductor with a cross-section of 1 circular mil and a length of 1 foot has a resistance of 12.9 × 10⁻⁶ ohms at 75°C.

The 75°C temperature is significant: it represents the typical operating temperature of a conductor under load. At higher temperatures, copper's resistivity increases (copper has a positive temperature coefficient of resistance), so actual voltage drop on heavily loaded conductors can be slightly higher than the formula predicts at room temperature. The 75°C value is appropriate for standard NM-B and THHN/THWN conductors sized for their 75°C ampacity column.

For aluminum conductors, K = 21.2. Aluminum has higher resistivity than copper, which is why aluminum conductors must be upsized when substituting for copper — typically by one wire size (e.g., 2 AWG aluminum in place of 4 AWG copper).

Circular Mils Reference — Common Wire Sizes

The circular mil (CM) area for each standard AWG wire size is tabulated in NEC Chapter 9, Table 8. The most common values for residential and light commercial work:

NEC 3% Recommendation — What It Actually Means

The NEC does not set a mandatory maximum voltage drop for branch circuits. There is no code section that makes a 3% or 5% limit enforceable on its own. The 3% figure appears in NEC Informational Notes (formerly called Fine Print Notes) and in Annex B, which is explicitly labeled "Informational Annex — not a part of the requirements of the NEC." It is a recommendation, not a requirement.

That said, the 3% recommendation reflects real engineering practice. Voltage drop above 3% on branch circuits causes measurable effects:

• Reduced light output. Incandescent lamps are most affected — a 5% voltage drop reduces light output by about 10% and changes color temperature. LED fixtures are less sensitive but may show reduced lumen output on long drops.
• Motor heating. Motors drawing rated current at reduced voltage overheat because the motor compensates by drawing more current, increasing I²R losses in the windings.
• Equipment underperformance and potential damage. Some electronic equipment has narrow supply voltage tolerances. Chronic undervoltage can damage motors and other loads designed for a specific voltage range.
• Nuisance tripping. Equipment with internal voltage monitors may fault or shut down on undervoltage conditions during high-load periods when drop is greatest.

Your local AHJ, specific equipment manufacturers, or specialized installations (healthcare, data centers) may impose stricter requirements. Always check equipment manufacturer specifications for minimum supply voltage in addition to NEC.

Worked Examples — Common Residential Circuits

Bedroom Circuit — 14 AWG, 75 ft, 15A

A bedroom circuit with a 75-foot one-way run, 14 AWG copper, 15A breaker, 120V. Using the design current of 80% of breaker rating (12A for a non-continuous circuit) for a realistic load estimate:

VD = (2 × 12.9 × 12 × 75) / 4,110 = 23,220 / 4,110 = 5.65V
VD% = (5.65 / 120) × 100 = 4.7% — over the 3% recommendation

At 50 feet with the same load: VD = (2 × 12.9 × 12 × 50) / 4,110 = 3.77V = 3.1% — borderline. For a 75-foot bedroom circuit, 12 AWG is the better choice.

With 12 AWG at 75 ft: VD = (2 × 12.9 × 12 × 75) / 6,530 = 23,220 / 6,530 = 3.56V = 2.97% — within 3%.

Garage Workshop — 12 AWG, 100 ft, 20A

A workshop receptacle circuit, 100-foot one-way run, 12 AWG, 20A breaker, 120V. Design current 16A (80%):

VD = (2 × 12.9 × 16 × 100) / 6,530 = 41,280 / 6,530 = 6.32V
VD% = (6.32 / 120) × 100 = 5.3% — significantly over 3%

With 10 AWG at 100 ft: VD = (2 × 12.9 × 16 × 100) / 10,380 = 41,280 / 10,380 = 3.98V = 3.3% — still slightly over. At 20A design current (100% — appropriate if the circuit feeds a continuous load): 5.0V = 4.1%. For long workshop runs, 10 AWG is recommended and 8 AWG may be warranted for runs over 100 feet with heavy tools.

EV Charger — 10 AWG, 150 ft, 16A charger (NEC 625.41)

An EV charger with a 16A nameplate, 125% continuous load rule requires a 20A circuit, 10 AWG minimum, 150-foot one-way run, 240V. Design current = nameplate amps = 16A on 240V:

VD = (2 × 12.9 × 16 × 150) / 10,380 = 61,920 / 10,380 = 5.96V
VD% = (5.96 / 240) × 100 = 2.5% — within 3% on 240V

Note: the same physical drop (5.96V) that is 4.97% on a 120V circuit is only 2.48% on a 240V circuit. This is the core reason why 240V is preferred for long runs and high-power loads — the same wire produces a lower percentage voltage drop at higher voltage.

With a 48A charger (requiring a 60A circuit, 6 AWG minimum) at 150 ft: VD = (2 × 12.9 × 48 × 150) / 26,240 = 185,760 / 26,240 = 7.08V = 2.95% at 240V — within 3%, but close. 4 AWG is warranted for 48A chargers on runs over 125 feet.

240V Circuits — Why Voltage Drop Is Lower

For any given wire size and run length, a 240V circuit has half the percentage voltage drop compared to a 120V circuit carrying the same current. The absolute drop (in volts) is the same — the formula doesn't change — but because the base voltage is doubled, the percentage is halved.

This is the fundamental reason high-power loads — ranges, dryers, water heaters, EV chargers, HVAC equipment — are wired at 240V. At the same conductor size, a 240V EV charger circuit can travel twice as far before hitting the 3% voltage drop limit compared to a 120V circuit with the same current. For a 48A charger on a 150-foot run, 240V operation keeps the drop within 3% with 6 AWG; the same current at 120V would be completely impractical.

Frequently Asked Questions

• Is the 3% voltage drop limit actually required by the NEC?

  No. The 3% figure appears in NEC Informational Notes and Annex B — both are explicitly non-mandatory. There is no NEC section that makes 3% voltage drop an enforceable requirement. It is a widely adopted engineering recommendation that reflects the point at which voltage drop begins to cause measurable problems with equipment performance. Local amendments or equipment specifications may impose stricter limits.

• Should I use load current or rated current in the formula?

  For design purposes, use the expected operating current of the loads on the circuit — not the breaker rating. The breaker rating is the overcurrent protection limit, not the design current. For a 20A circuit supplying a workshop, the realistic continuous operating current might be 12–16A rather than 20A. For EV charger circuits, use the charger's nameplate current (the actual continuous draw). Sizing for 100% of the breaker rating on a circuit that typically runs at 60% load adds unnecessary cost without meaningful benefit.

• Does voltage drop affect GFCI operation?

  No — GFCI devices detect current imbalance between the hot and neutral conductors, which is independent of the supply voltage. A GFCI will trip at the same ground fault current threshold regardless of how much voltage drop exists on the circuit. Excessive voltage drop is a power quality concern, not a safety protection concern for GFCI operation.

• What is the maximum voltage drop for a motor circuit?

  NEC 430.52 and Article 430 govern motor circuits and do not specify a maximum voltage drop. However, motor manufacturers typically specify a minimum voltage tolerance of ±10% of nameplate voltage. For a 120V motor, that means a minimum supply of 108V at the motor terminals — far more permissive than the 3% commonly targeted for lighting. For starting conditions (locked-rotor current), the instantaneous voltage drop during motor starting can be much higher and may affect other loads on the same circuit, which is a separate consideration from steady-state voltage drop.

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